The present invention provides a multi-functional apparatus and process for biological wastewater treatment. The present invention addresses multiple critical environmental needs, including energy efficient treatment of wastewater, abatement of greenhouse gases (GHGs) produced from conventional wastewater treatment processes, CO2 capture from CO2 generators, and biomass production for renewable energy, fertilizer, feed additive, bio-plastics, cosmetics, pharmaceuticals, fabrics, bio-fuels, and other uses.
Wastewater treatment has grown significantly from its origins for treatment of metropolitan sewage. Environmental protection regulations require treatment of effluent from wastewater generators prior to drainage into a common waterway. Treatment processes now exist to meet these regulations, but the methods create significant GHGs, and are complex, expensive and energy intensive. Bacteria-based treatment processes were developed when energy costs were low and there were no concerns about climate change. Obviously, that is not the case today. Two major problems with current wastewater treatment technologies are their large energy consumption and large carbon footprint. According to the U.S. EPA., wastewater treatment plants (WWTPs) account for 3% of the entire U.S. electrical demand and generate 3.4% of all GHG emissions in the U.S.
The two most widely used processes for wastewater treatment are the activated sludge and bio-film systems. There are over 16,000 WWTPs in operation in the U.S. Of these, 6,800 are activated sludge municipal wastewater treatment plants which require 1.3-2.5 MWh per every million gallons (MG) treated. There are over 2,500 municipal bio-film systems in the U.S. which require 0.8-1.8 MWh per MG. In addition to activated sludge and bio-film systems, there are over 5,100 pond type wastewater treatment systems in the U.S. The energy requirement for pond systems is typically lower at 0.4-1.4 MWh per MG but these types of systems are not capable of meeting current direct discharge regulatory requirements. Pond systems also require large amounts of land and have large carbon footprints because they generate methane from anaerobic digestion in the bottom of the ponds. For this reason, some regulatory agencies will no longer allow these types of systems at all. Regulatory agencies will soon also require nitrogen and phosphorous removal at most municipal WWTPs. Nitrogen and phosphorous removal is very difficult to achieve with an activated sludge or bio-film system and would add significantly to the capital cost of WWTPs as well as increase their energy consumption and GHG emissions.
Conventional wastewater treatment involves three process stages, called primary, secondary and tertiary treatment, followed by sludge processing. In the primary stage, paper, plastics, and large solid objects are separated from the wastewater stream by coarse or fine mechanically or manually cleaned screens. Additional solids, grease, and scum are removed utilizing primary clarifiers or mechanical filters designed to replace primary clarifiers.
In the conventional secondary stage, organic material is digested using indigenous, water-borne and predominantly non-phototrophic bacteria. Secondary treatment systems are generally classified as either bio-film or suspended growth. Bio-film treatment processes include trickling filters and rotating biological contactors (RBCs) where the biomass grows on media and the sewage passes over its surface. Bio-film systems are not capable of efficiently growing algae or phototrophic bacteria because of mechanical problems and clogging. RBCs are typically covered to prevent exposure to sunlight with trickling filters that are generally vertical in geometry with a very small surface area exposed to sunlight. In suspended growth systems—such as activated sludge and membrane bioreactors (MBRs)—the biomass is well mixed with the sewage and can be operated in a smaller space than bio-film systems that treat the same amount of water. However, like the bio-film systems, suspended growth systems are not capable of growing algae or phototrophic bacteria due to the high concentration of bacteria maintained in the system, typically referred to as mixed liquor suspended solids (MLSS) concentration.
In a conventional WWTP, ammonia is converted to nitrates through another bacteria based process called nitrification. This process can be performed in a “separate stage nitrification” process or combined with the secondary treatment process. The treated water is finally disinfected using chlorination or UV disinfection prior to discharge to a body of water.
The sludge generated from the wastewater processes is accumulated in sludge processing tanks where it is broken down or digested by aerobic or anaerobic processes. After digestion, the sludge is dewatered, dried, and hauled to a landfill or land applied. The sludge handling at conventional WWTPs is extremely energy intensive, requires chemicals for dewatering and sludge stabilization, and uses fossil fuels to process and transport the sludge for final disposal. The sludge, regardless of whether it is taken to a landfill or land applied, is converted by bacteria to greenhouse gases. Sludge that is land applied also creates public health hazards from fecal and pharmaceutical contamination of food crops. Several states and nations are moving towards banning the practice of land applying human feces altogether.
A second critical environmental need is the reduction of the carbon footprint associated with conventional wastewater treatment processes and the capturing of CO2 from CO2 generators. All conventional biological wastewater treatment and sludge digestion processes convert the organic and inorganic components in the wastewater to greenhouse gases. Conventional WWTPs simply convert one form of pollution into another—solid to gas. The conventional secondary treatment process creates CO2 gas from bacterial respiration. The conventional tertiary stage process creates nitrous oxide (N2O) from the nitrification process, a GHG that is 310 times more potent than CO2 (Kyoto protocol). The sludge digestion processes creates methane (CH4), a GHG that is 21 times more potent than CO2 (Kyoto protocol).
Current ideas for carbon sequestration include pumping carbon dioxide underground and capture in algae systems. The main problem with pumping CO2 underground is that the energy requirements make this approach impractical to implement. Another problem is the risk that the gases will escape to the surface. There have been documented cases where naturally occurring CO2 gases escaped from underground to the surface of the earth killing all of the humans and animals in the surrounding area. Pumping CO2 underground is the equivalent to dumping our wastes into the ocean. We have no idea what the future consequences of such actions would cause.
Carbon sequestration via algae based systems is also impractical. The most efficient algae production rates from various algae production technologies currently being tested range from 50-100 dry tons of algae per acre per year. It is known that algae is approximately 50% carbon and uses approximately 1.9 lbs of CO2 for every 1.0 lb of algae produced. It is also known that 1.0 lb of coal typically creates approximately 2.7 lbs of CO2. Therefore, it can be calculated that 1.42 lbs of algae is required to sequester the CO2 generated from the combustion of 1.0 lbs of coal. Using the highest algae production rate indicated above, it can also be calculated that a 1.0 acre algae production system could sequester the CO2 generated from 70.4 tons of coal per year. According to the U.S. Energy Information Administration, the U.S. currently consumes 1,129 million tons of coal per year. In order to sequester the CO2 generated from this coal, 1,603 million tons of algae would be required which translates to 16 million acres or 25,000 square miles of land, or roughly the entire state of Virginia.
Another critical environmental need is to provide a cost effective and reliable biomass production system. The biomass produced from the system can be used as a feedstock for renewable energy production, fertilizer, and other useful products. The need for a renewable energy source has become particularly acute and the subject of widespread concern. For example, fossil-fuel based energy (gas and oil) are known to be finite. While the debate rages as to exactly how finite is “finite”, much evidence suggests that worldwide oil production will peak in around 2010, and that the oil supply will end as early as 2035 but no later than 2060. Nevertheless, there is no question that the fossil fuels will be depleted.
Awareness of the limited life of fossil fuels has prompted significant research and development for renewable energy sources. Much research has been devoted to alternative energy sources, such as solar, wind and biomass. However, these alternative energy sources cannot cost effectively and reliably produce electricity and do not appear to have the near-term capability of satisfying the need for petroleum-type fuels—i.e., gasoline and diesel fuels. Research in the 1980s focused on developing gasoline and diesel fuels based on renewable resources, such as corn-based ethanol and bio-diesel. Most bio-diesels are based on food crops, such as soybeans, which require a significant amount of energy to grow and harvest. Moreover, the food crops themselves must be devoted to the production of biofuels.
Research conducted from 1980-1996 by the U.S. Department of Energy established algae as a source of biofuels. Biofuel can be produced from algae by digestion for methane or hydrogen fuels, lipid extraction for bio-diesel, and distillation for ethanol. In addition to its benefits as a precursor to biofuels, algae has been developed for other uses, such as an organic fertilizer which could be used as a replacement to fertilizers produced from natural gas.
Biodiesel has been investigated by the U.S. Department of Energy (DOE) as part of its “Aquatic Species Program” that began in 1978. Funding for this program was eliminated in 1995, but growing concerns over non-renewable fossil fuels has prompted the DOE to reopen the program due to the growing interest in this seemingly infinite and renewable source for biofuels. The DOE's approach has been to create algae ponds or “raceways” near factories that generate waste CO2. The waste CO2 and other nutrients are injected into water circulating around a racetrack shaped pond. Algae growing in the circulating water feeds on the CO2. The algae are eventually diverted from the pond for further processing as a biofuel. Thus, the DOE focus has been on artificially creating a growing environment for algae by recycling waste CO2 from a factory or a coal-fired power plant. Of course, one significant limitation of this technology is that it is tied to a source of waste CO2. Another detriment is that this proposed technology requires a large raceway pond, and ultimately a large amount of dedicated land in order to support enough algae to accept the waste CO2 and to produce a meaningful amount of algae for biofuel production. Since the algae require exposure to sunlight for growth, the ponds must be shallow, which means that the surface area of the pond must be very large to support the algae colonies. The large size of the pond also means that the useful “season” is limited in certain locales and climates due to freezing of the pond.
There currently are no algae production systems in the world that can replace a conventional extended aeration system and achieve the same level of treatment. Some algae production systems have been proposed on WWTP effluent but that provides little benefit to the treatment plant owner because the wastewater has already been cleaned. In fact, it creates a major liability and risk to the owner because of the potential re-contamination with algae solids with certain types of algae production systems. High rate algae ponds have been used for wastewater treatment but pond systems are not capable of meeting current regulatory discharge requirements and require very large amounts of land as compared to mechanical WWTPs. One reason why current algal production systems cannot be used to treat wastewater is due to the fact that algae cannot use organic carbon as a carbon source. Carbon in wastewater as it enters the WWTP is in the form of organic carbon which is essentially useless to the algae at that point. The organic carbon must first be converted to CO2 by bacteria through respiration. Current algae production systems lack a bio-media component to provide the growing means for the bacteria required for this conversion to take place. Another problem with growing algae in any kind of pond is that only in the top ¼ inch or so of the water does the algae receive enough solar radiation. Thus, the ability of the pond to grow algae is limited by its surface area, not by its volume.
Algae produces oxygen necessary for aerobic bacterial growth and bacteria produces CO2 needed for algal growth. The only external input to fuel this symbiotic relationship is sunlight. This strategy was first successfully implemented in open lagoons and wetland treatment facilities. These systems had obvious limitations, such as land space, geography and topography, water clarity, etc. In addition, the lagoon systems were prone to algae blooms that would overrun and clog the systems. These limitations led to the development of the algae raceway in the 1970s. The algae raceway is essentially a flume in which nutrient-rich water is allowed to course while exposed to sunlight. The resultant algal biomass is harvested by mechanical means. One significant detriment of the algae raceway is that it requires a large surface area for adequate exposure to sunlight. In addition, the raceway requires a shallow water level to function, which inherently limits the volume and flow of wastewater that can be treated by any particular raceway facility. Still another problem with ponds and raceway systems is predation by animals and insects. The larvae of some insects feed on the algae and can consume the entire crop of algae almost overnight.
Closed loop bioreactors have also been developed for algae production. Closed loop bioreactors are typically transparent plastic tubes, plastic bags, plastic sheets, resins, glass or any material that allows light to penetrate. The proposed advantage of closed loop bioreactors is that the system allows more control over the algae and growing conditions because it is not open to the environment. One of the disadvantages of closed bioreactors is that as the algae increases in the container, the uniform light distribution throughout decreases due to the light being absorbed by the algae. The outermost layer of algae in the reactor get too much light and the inner layer of algae do not have enough light. Algae also produces organic compounds that coat the closed bioreactor and slowly reduce the ability of light to penetrate the bioreactor. The bioreactor material has to be either cleaned or replaced increasing operation and replacement costs.
Currently proposed closed bioreactors cannot be used with typical exhaust blowers found at coal plants due to the water depth of the reactors which creates too high of a pressure head for the blowers to pump against. Still another problem in closed loop bioreactors is gas buildup. Since the reactors are completely closed and CO2 is being compressed into the growing container, gas concentrations can build to levels that are toxic to the algae and that are detrimental to the equipment. Another problem with closed loop bioreactors is the amount of energy required to move the water through the system, especially the vertical algae growing systems. The amount of energy required to pump the water through the system exceeds the energy obtained by the algae produced. This is essentially the same problem ethanol plants have encountered by consuming more energy than energy produced. Heat buildup is another problem with closed loop bioreactors. Ultimately, closed loop bioreactors are poor substitutes for conventional wastewater treatment processes.
The present invention solves these challenges in wastewater treatment, GHG emissions reduction, and algae and biomass production.